Combustion of coal



Sept. 5, 1933.

C. M. BUCK COMBUSTION OF COAL Filed Aug. 16 1952 Fa x 4 Patented Sept.5, 1933 UNITED STATES PAITENT OFFICE Application August 16, 1932. SerialNo. 629,063

4 Claims. (Cl. 110104) This invention relates to the combustion of coaland is primarily concerned with improvements in the art of burningpulverized coal.

The nature, objectsand advantages of the 5 invention can best be'understood after a consideration of the following comments with respectto various characteristics of coal or coals suitable for burning in afinely comminuted condition as vwell as the methods or procedures atpresent in more or less common commercial use.

For several reasons which need not be discussed, pulverized coal isalmost universally milled from various soft coals such as bituminouscoals. In view of the above, the following description refers moreparticularly to such soft coals and their treatment and burning inaccordance with the novel method hereinafter more fully disclosed. 4

As is well recognized by many engineers and .authorities on combustionof carbonaceous maserials, coal, as it is heated from normal atmospherictemperatures up to and through combustion temperatures, undergoesvarious physical and chemical changes, some phases of which take placebetween very accurately determinable points in the temperature scale.The temperatures at which such changes occur are dif-'- ferent withdiiferent coals, but these may be determined by observation of the coalitself or analysis of certain gases and the like which may be given off.

Thus, with some soft coals, for example, bituminous from the Pittsburghdistrict, treatment at temperatures up to approximately the boilingpoint of water, for example, 212 R, will result only in a dryingoperation which usually involves mere elimination of surface moisture,i. e., moisture or water carried on the surface of the pieces of coaland in the cracks and crevices thereof. In addition, all coals includein this range will serve the purpose of reducing the moisture content sothat a given weight of coal has a considerably higher calorific valuethan before the heat treatment, these temperatures, for any given coal,are considerably above those at which exothermic combustion begins,

i. e., combustion or burning without the external application of heat.In' other words, at some point in this range the coal actually burns andproduces suflicient heat in and of itself to continue its thermaldecomposition and exothermic oxidation. I

In addition to the foregoing, some coal drying DIOCQSSGSlIll/GIVGheating the coal as by means of an inert gas which is at a temperaturesomewhat above the boiling point of water, but it is generallyrecognized that for drying purposes, the temperature of the coil itself,i. e., the mass or bulk of the coal lumps or pieces should not be raisedsubstantially beyond the boiling point of water, this temperatureusually being maintained materially below 250 F. Heating of the mass orbulk of the coal throughout, however, should not be confused withsurface heating thereof intended only to dry up surface moisture, andwhile drying processes quite frequently employ gases at a temperatureconsiderably above the boiling point of water, the drying processes areso designed as to avoid heating of the coal throughout to such hightemperature, as by relatively rapid flow of the coal through the heatingchamber.

There are, of course, additional processes involving heating of coal,such as those employed in the production of coke and other products,which necessitate destructive distillation or exothermic oxidation evenbeyond that referred to above in connection with known processes forincreasing the calorific value of a given weight of coal by eliminatingcombined moisture. Such processes, however, also produce materialthermal decomposition or oxidation.

In accordance with the more usual method of procedure where it isdesired to burn finely comminuted or pulverized coal insuspension, somesoft coal, for example, bituminous, is heat treated to remove surfacemoisture, then pulverized, and ultimately fed'to the furnace throughburner nozzles, a blast of air being provided to carry the fineparticles in suspension, either from a point of storage or more or lessdirectly from the Dulverizing mill.

The foregoing procedure which has become commonly accepted, presents anumber of very serious disadvantages, among which the following arementioned.

Since coal, dried and pulverized in the manner above referred to, whencombined with sufficient air for purposes of transportation insuspension, forms an extremely explosive mixture, apparently due, atleast in part, to the presence in the coal of free active gases,ordinarily imprisoned or occluded, which have been released duringpulverization, the handling of it and use thereof in power plants andthe like is very dangerous indeed. In addition, even when being storedin bins, the increase in the exposed surface area in the coal, as aresult of pulverization, quite frequently results in autogenouscombustion.

A still further characteristic of coal treated and pulverized inaccordance with prior practice is an extremely high rate of flamepropagation when carbureted with material quantities of air. In view ofthe characteristics referred to, especially the high rate of flamepropagation evidently resulting from the presence-of free active gases,the amount of air which is mixed with the' coal and used to transportthe same into the combustion space, in accordance with prior practice,represents only a small fraction of the total amount necessary tocomplete combustion of the fuel. This small percentage of air,furthermore, has very high linear velocity for the purpose of avoidingburning back of the flame into the burner nozzles. In other words, inaccordance with the commonly accepted method of procedure, it isessential to employ a very high velocity air blast to carry thepulverized coal into the combustion space. The additional air necessaryto complete combustion of the fuel is admitted secondarily into thecombustion chamber independently of the burner nozzles, by natural orforced draft. As a result of the high velocity fuel admission, with onlya fraction of the total air necessary, the overall dimensions of thecombustion space must be relatively great in order to ensure completecombustion of the fuel before entry of the fuel and flame stream intothe furnace off-take.

Another disadvantage arises from rapid and serious erosion of therefractories employed, as well as of metallic elements, such as tubeswith which the combustion chamber may be lined. Still further, since amajor portion of the combustion air is admitted secondarily, throughopenings spaced from the burner nozzle and usually spaced along the pathof travel of the fuel and flame stream, the mixture of the air with thepowdered. coal is very irregular.

Because of improper or irregular admixture of the coal and air andbecause of certain characteristics of coal more fully consideredhereinafter, the common practice of burning powdered fuel requiresadmission of substantially more air into the furnace than is actuallynecessary for complete combustion, with the result that the size of thecombustion space is unnecessarily large for this reason also. Slaggingdifficulties at the bottom of the furnace have also been encountered.

Turning now to the present invention, it

I should be noted that, in general, the objects are involved ineliminating, or at least, ameliorating the foregoing difficulties anddisadvantages.

More specifically the'present invention has in view a method for theburning or combustion of coal involving feeding of substantially all ofthe air necessary for complete: combustion of the coal with the coalas-it is delivered or discharged into the combustion space.

A further object of considerable importance is involved in certainfeatures of my process or method which make possible complete combustionof the fuel in the combustion chamber with less air than has beenrequired heretofore.

I have found that by heating the coal throughout, preferably prior tocomminution or pulverization, up to a temperature materially above theboiling opint of water, but not beyond the point at which activeexothermic oxidation or thermal decomposition commences, certainchanges, principally physical, are brohght about. For bituminous coalswhich I refer to by way of example,

,I have found that temperatures between approximately 250 F. and 480 F.are suitable, and I preferably effect this heat treatment either throughthe medium of, or in the presence of, inert and non-combustible gases,such as stack gases discharged from a furnace.

Since the physical and chemical characteristics of coal varyconsiderably, the point at which exothermic combustion commences shouldbe determined in each instance. This may be accomplished in a variety ofways, as by observing the coal undergoing treatment and noticing at whattemperature certain changes in color are brought about, these changesprobably being caused by the formation of oily substances on the surfaceof the coal. Another indication may be obtained by observing at whattemperature the treated material smokes when discharged from the heatingchamber (containing the inert gases) into the atmosphere.

While the present method involves heating of the coal, within thetemperature range indicated, throughout the entire mass or body of thecoal, I preferably terminate the period of heat treatment (especiallywhere the temperature employed approaches the point of active thermaldecomposition) very quickly, if not instantly, after the coal has beenheated throughout. With this in mind, in accordance with the preferredprocedure, the lump coal is first crushed to pieces approximating aboutone-quarter inch in diameter, and although my heat treatment may beemployed with coal which has already been pulverized, I find itespecially desirable to follow the method indicated as preferable sincethe coal may much more readily be pulverized after the treatment. Onereason for the improvement in pulverization which results is the factthat the coal is expanded and becomes quite porous, the amount ofexpansion usually being in the neighborhod of about 10%. v

The foregoing heat treatment, of course, ac-

complishes removal of surface moisture which i has been heretoforebrought about by the usual drying processes, but the major importance ofmy method results from certain other changes which take place.

The treatment in the above range, although it does not remove much ofthe combined moisture, does result in'the liberation of certain occludedgases, for example, methane, ethane, carbon dioxide, carbon monoxide,nitrogen, and possibly others in small quantities, some of which arehighly inflammable. Moreover, as above brought out, the coal expands andbecomes porous, and, as a result, it absorbs or. picks up substantialquantities of the inert gases in the presence of which the treatment iseffected. It also seems clear from such tests as it has been possible tomake that certain chemical affinities are also satisfied.

In any event, whatever may be the physical or chemical causes, I havefound that the reactions and physical changes which are brought aboutduring this phase of combustion of the coal, i. e., that portion of theendothermic phase which extends up to the point at which exothermicorthermal decomposition commences, provide a fuel which still retains amaterial percentage of combined moisture (by way of example, with somebituminous coals about 7% of moisture), and which, when mixed withsubstantially all the air necessary to complete its combustion, i. e.,carry on the combustion through its exothermic phase, burns in a greatlyimproved and much more eficient manner.

While the endothermic phase of combustion (that is, the. phase duringwhich externally applied heat may be absorbed by the coal) extendsbeyond the temperature at which thermal decomposition or activecombustion sets in, by carrying on or completing the combustion processup to the point of active oxidation (at which point the reactions in thecoal itself produce suihcient heat to create a tendency for the coal tocarry on and complete its burning without the application of externalheat), and, subsequent to. the completion of combustion to the extentreferred to, mixing the fuel with the remaining air necessary, I amenabled to introduce the fuel at relatively low velocity as comparedwith prior practice without the flame burning back, principally becauseof the absence from the coal of any free active gas sufficient. toproduce a self propagating flammable mixture with air, and further toutilize a combustion space of materially reduced size as compared withthat heretofore required.

The carrying on of combustion in its several phases partly externally ofthe furnace and partly after introduction of the fuel into thecombustion space, therefore, makes possible this use of a reduced spaceto burn a given quantity of fuel. In other words, I introduce the fueland air at relatively low velocity and by virtue of this fact materiallyreduce erosion of refractories and the like, and, additionally, providefor com-= pletion of combustion of the fuel during a relatively shortfuel and flame stream travel.

of the coal in the furnace are carried out in the combustion spaceitself, this procedure necessitating the feeding of excess quantities ofair, by

following my procedure, a reduced quantity of air per pound of fuel maybe fed therewith into the combustion space and still produce efficientand complete combustion.

Thus my improved method adapts the burning of pulverized coal to usesand conditions to which it has notbeen suitable heretofore, as inlocomotives, the combustion chambers of which must, of necessity, be ofrelatively restricted dimensions.

Other advantages of the present method will now be apparent. Forinstance, since I feed all or substantially all of the air necessary forcombustion in the furnace space with the fuel as it is delivered ordischarged into the furnace, I am enabled to obtain greatly improvedadmixture of the fuel and air as will be apparent when this procedure iscontrasted with prior methods according to which the major part of thecombustion air is introduced into the furnace secondarily through spacedopenings in the walls of the chamber. Moreover, since I introduc'ethefuel in a relatively slowly moving current of air consid- -erably lesspower is necessary to create the air draft for a given" volume orquantity thereof, this being of very material importance in view of thefact that the power which would be necessary to.

create a blast of air of sumcient volume to complete combustion of thefuel at velocities necessarily employed with ordinary raw pulverizedcoal would be entirely impractical.

Since pulverized coal feeders adapted to deliver all of the airnecessary to complete combustion are not of common knowledge or readilyavailable, the present application is accompanied by a drawingillustrating a feeder suitable to carry out the purposes I have .inmind. As brought out more fully below the drawing also includes ashowing of coal heating and pulverizing equipment such as that referredto above. In this drawing:

Fig. l is a vertical cross sectional view of the feeder;

Fig. 2 is a side elevational view of the apparatus shown in Fig. l, withportions thereof broken away in order to disclose others lying behind;and Figure 3 is a diagrammatic showing of coal crushing, heating andpulverizing equipment of a type suitable to carry on certain of theprocess steps above referred to. i

Referring first to Figure v3, as hereinbefore crushed to piecesapproximating about id" in diameter, this being accomplished bydelivering the lump coal through hopper A to any suitable crusher B. Thecrushed coal may be delivered to the retort or heating chamber C througha conduit D and, upon discharge of the coal from suitable for effectingthe treatment. The coal may be conveyed. away from the pulverizer Fthrough a conduit G and finally, of course, the coal is delivered to afeeder mechanism such'as that shown in es 1 and 2 and described below.

As before mentioned the coal may be heated in the chamber C by fluegases discharged from furnace H through connection I. A discharge orvent pipe J may serve to carry the gases away from the chamber C.

ing 4. The hopper has a discharge throat 5 in which I preferably mount achurning or heater wheel 6, the throat being slotted as at 7 to providefor the delivery of a relatively thin and wide sheet or stream ofpowdered coal into the sub h the feeder illustrated I employ a hopper orbin 3 adapted to be charged'through the openjacent and associated fancasing indicated -generally by the numeral 8.

The fan device includes a fan rotor 9 formed is spaced a relativelygreat distance from the periphery of the fan at the point indicated at12, from which point the casing progressively ap- 5 proaches the fanperiphery to the point 13. A

screened air inlet is provided peripherally of the fan as indicated at14 and a discharge conduit 15 for delivering the fuel to the combustionchamber is extended away from the fan housing between the portion of thewall designated by the numeral 13 and an additional wall portion orpart16, the latter being of relatively small dimention circumferentially ofthe fan and disposed in close proximity to the periphery of the fanblades. vWith this arrangement, as seen in the drawing, the air inlet 14and the discharge conduit 15 are disposed relatively close to each otherat the same side of the fan, there being only a relatively small wallpart 16 interposed therebetween.

The air inlet opening, the width of the feed conduit 15 and the fueldischarge slot 7, are all preferably substantially equal in width to theaxial length of the fan. If desired, elements 1'? in the nature ofarcuate bars may be disposed toward the lower end of the bin 3 so as toprevent arching of the fuel therein, and packing of the fuel, especiallyin the lower portion of the bin, is further avoided by the use ofaerating tubes 18.

In operation, therefore, the fuel, in being delivered downwardly throughthe bin and past the feeder wheel 6, is discharged into the fan housingin a thin but wide sheet, andas the fan rotates in the directionindicated by the arrow this fuel is entrained in the air drawn inthrough the inlet 14 and blown around the volute-like fan casing to thepoint of tangential discharge into the duct 15. As the fuel and airadvance through the progressively narrowing volute-like fan passage,centrifugal force, acting on fuel and air disposed between adjacent fanblades, throws the mixture outwardly against the casing from which it isagain reflected or deflected" toward the fan rotor and picked up. Thisoperation is repeated until the point of discharge is reached, at whichtime very thorough and complete admixture of the fuel and air isprovided. Upon discharge of the air and fuel from between thefan bladesunder the action of centrifugal force, a negative or reduced pressure isproduced and this condition is maintained until the spaces between theblades move upwardly beyond the wall portion 16. In view of the factthat the wall portion 16 is of relatively small arcuate dimension, andfurther since it is disposed in close proximity to the periphery of thefan blades, a very substantial flow of air inwardly through the inletopening 14 is produced and the incoming air, as above brought out, mixeswith the descending fuel in the manner described.

By this feeder device I am enabled to thoroughly admix all orsubstantially all of the air necessary for combustion with the fuel andto discharge this mixture into the duct 15 to be conveyed thereby intothe furnace combustion space.

The arrangement of the several parts of the device, furthermore, aresuch as to deliver the necessarily large quantity of air at the desiredrelatively low velocity.

While other feeder devices may be developed to accomplish the purpose Ihave in mind, I prefer the foregoing embodiment, especially since thestructure thereof, having a fan, a fuel inlet slot, an air opening and adischarge conduit, all of substantially equal width, provides foruniform and thorough admixture throughout the entire width of a sheeteven of relatively great width.

In conclusion, I desire to point out that by following the method orprocedure herein disclosed,

the exposed surface area of the fuel is materially increased, not onlyas a result of theexpansion of the fuel but further since there is asubstantial increase in ultrafines and pulverization to a much finerstate is made possible, and when the fuel is delivered into the furnaceit is ready for immediate gasification and active combustion. It is,therefore, of especial advantage'to feed substantially all of the airnecessary to complete its combustion along with the fuel at the time ofits admission.

The present application is a continuation in part of my copendingapplication Serial No. 503,- 169, filed December 18, 1930, issuedJanuary 10th, 1933 asPatent No. 1,893,857.

I claim:

1. The method of burning coal of the softer grades which includes makingexternal application of heat to the coal until its temperature is raisedthroughout its mass to a point, between about 250 F. and 480 F.,approximating but not above the point at which active or exothermiccombustion of the particular coal commences to render the coalsubstantially non-explosive, pulverizing the coal, and feeding the heattreated and pulverized coal to the combustion space of a furnaceinsuspension in substantially all the air necessary to complete itsexothermic phase of combustion.

2. The method of burning coal of the softer grades which includes makingexternal applica-' tion of heat to the coal until its temperature israised throughout its mass to a point between approximately 250 F. and480 F. to carry on certain phases of its endothermic combustion torender the coal substantially non-explosive, pulverizing the coal, andfeeding the heat treated and pulverized coal to the combustion space ofa furnace in suspension in substantially all the air necessary tocomplete its exothermic phase of combustion.

3. The method of burning coal of the softer grades which includes makingexternal application of heat to the coal in the presence of inert gasesuntil its temperature is raised throughout its mass to a point betweenapproximately 250 F. and 480 F. to carry on certain phases of itsendothermic combustion to render the coal substantially non-explosive,pulverizing the coal, and feeding the heat treated and pulverized coalto the combustion space of a furnace in suspension in substantially allthe air necessary to complete its exothermic phase of combustion.

4. The method of burning, coal of the softer grades which includesmaking external application of heat to the coal in the presence of inertgases to carry on certain phases of the endothermic combustion of thecoal, the application of heat being suflicient to bring thetemperatureof the body or mass of the coal to a point between about 250 F. and 480F. and in the neighborhood of but not above the point at which thermaldecomposition or exothermic oxidation of the particular coal comnfencesto render the heated coal substantially non-explosive, pulverizing thecoal, and feeding the pulverized and partially burnt coal into thecombustion chamber of a furnace in suspension in at least a majorportion of the air necessary to complete its exothermic phase ofcombustion.

CHARLES M. BUCK.

